Columnar optical properties of tropospheric aerosol by combined lidar and sunphotometer measurements at Taipei, Taiwan Wei-Nai Chen a, * , Yi-Wei Chen b , Charles C.K. Chou a , Shih-Yang Chang c , Po-Hsiung Lin b , Jen-Ping Chen b a Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan b Atmospheric Science Department, National Taiwan University, Taipei, Taiwan c Department of Public Health, Chung Shan Medical University, Taichung, Taiwan article info Article history: Received 9 December 2008 Received in revised form 19 February 2009 Accepted 26 February 2009 Keywords: Lidar Sunphotometer Aerosol Lidar ratio Angstrom exponent Depolarization abstract Vertical extinction profiles and columnar optical properties (optical depth, Angstrom exponent, lidar ratio, and particle depolarization) of aerosols were obtained by simultaneous measurements with a depolarization lidar and a sunphotometer at Taipei, Taiwan from February 2004 to January 2006. Columnar optical depths are high in Feb–Apr (0.61–0.75) by sunphotometer measurements. Lidar measurements show the contribution of aerosols in the free atmosphere on columnar optical depths are about 44–50% in Feb–Apr and about 26–37% in other months. Back-trajectory analyses and depolariza- tion measurements show almost all of non-spherical aerosols originated from Northwest China which indicate Asian dusts frequently transported to Taipei from dust source regions in the free atmosphere. Aerosols with depolarization lower than 5% are found mostly originated from South China or Southeast Asia. Good correlations between columnar lidar ratio, particle depolarization, and Angstrom exponent are found for cases that columnar water vapor less than 1.5 cm. The effect of water vapor on particle depolarization is briefly discussed. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction East Asia is characterized by high and rapid growing anthro- pogenic emissions resulting from the high population and energy consumption in the region. These gaseous and aerosol components exert an essential influence upon the atmosphere modifying its structure, properties and components. Both natural and anthro- pogenic aerosols can affect climate directly by solar radiation scattering and absorption (Charlson et al., 1992) and indirectly by their abilities to nucleate cloud droplets (Twomey, 1991). In Taiwan, a major natural component of atmospheric aerosol is mineral dusts from arid and semi-arid regions of Mainland China (Liu and Shiu, 2001) and the major anthropogenic components are sulfate and biomass-burning smoke from local sources or Southeast Asia (Wang et al., 2007). Aerosol optical depth is a main parameter of the aerosols that has significant impact on the climate. Light scattering and absorption by aerosols cause reduction in solar radiation and influence visual air quality, thereby adversely impacting visibility and the quality of the atmosphere (Seinfeld and Pandis, 1998). Thus, the characterization of aerosol properties is crucial to understand their influence on the Earth-atmosphere radiation budget. Furthermore, the microphysical characteristics of aerosols can be classified according to their geographical origin such as urban, rural, marine or continental, and also to local meteorological conditions (Mu ¨ ller et al., 2007). In this paper, we present the intensive (independent on concentration) columnar optical prop- erties of aerosols such as optical depths, Angstrom exponents, extinction-to-backscatter ratios (so called lidar ratio), and particle depolarizations over Taipei determined by lidar and sunpho- tometer. Lidar ratio is usually treated as one of characteristics of particle (Mu ¨ ller et al., 2007) which ranges from 20 to 80 for urban aerosols, 45–65 sr for biomass burnings, and 30–40 sr for Asian dusts (Ansmann et al., 1992; Takamura et al., 1994; Anderson et al., 2000; Liu et al., 2002). Angstrom exponent is a good indicator of the size of particles (Schuster et al., 2006). Higher values (greater than 2) are typically observed for accumulation mode particles and lower values (near 0) have been observed for coarse mode particles such as dusts (Wang et al., 2004). The depolarization was widely considered as indicator of the non-sphericity of particles such cirrus and dusts (Sassen, 1991; Iwasaka et al., 2003). In Taiwan, distinct correlations between columnar optical depths and concentration of aerosols observed in Taipei and Tainan had been noticed might be owing to non-vertically well-mixed tropospheric aerosols (Shiu et al., 2006). Therefore, to obtain aerosol vertical profile is important in understanding the variation * Corresponding author. Tel.: þ886 226539885; fax: þ886 227833584. E-mail address: wnchen@rcec.sinica.edu.tw (W.-N. Chen). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.02.059 Atmospheric Environment 43 (2009) 2700–2708